We are investigating the chemistry and physiology of transmitter release from a photoreceptor. The receptors of the giant barnacle, Balanus nubilus, have been chosen because they are relatively large and because both they and cells postsynaptic to them are accessible for intracellular recording and chemical manipulation. This system is of particular interest because, as with vertebrate rods and cones, the receptors conduct passively, and a hyperpolarization of the receptor leads to a deplorization of postsynaptic cells. We have traced the individual receptors into the supraesophageal ganglion and observed their terminal ramifications with the light and electron microscope; intra-cellular recordings made from these terminal regions show that membrane potential changes there in response to light are what would be expected from passive decay of the change at the cell soma. Although most of the ganglion cells respond only to light OFF, we can identify visually and record from cells near the receptor terminals which respond to light ON as well. These cells hyperpolarize to light ON and depolarize to light OFF; they appear from our evidence to be directly innervated by the receptors.